The invention relates to a method of operating an interactive image display system. The invention also relates to an image source device for implementing such a method.
1. Field of the Invention
From the JPEG and MPEG standards (see e.g. communications of the ACM, april 1991 Vol. 34 no. 4 pages 31-44 and pages 47-58) it is known to compress images. These standards make it possible to reduce the amount of memory used to store images, for example in a video recorder or on an optical disk. After reading the image signal from the memory, it can be transmitted in its compressed form to a remote location. This occurs for example when a remotely located server providing video on demand serves to a user terminal such as a television set, possibly with a set-top box. In the user terminal the compressed signal is decompressed and displayed. The use of compression reduces the bandwidth required for transmission via the link between the image source device and the user terminal.
User terminals can be provided with a user control input, for example a handheld remote control unit, for entering user commands for controlling the functioning of the image source. Controlling the functioning may involve for example selection of sound volume, selection from a number alternative images or image sequences, selection from alternative language channels, controlling fast forward operation of a videorecorder etcetera.
The image display system can assist the user by providing information about available commands. The content of this information may be adapted after a command is actually entered, to show its effect and/or subsequently assailable commands. As this information depends on user actions, it is interactive. This information is generated by the image source device and is transmitted to the user terminal.
The MPEG standard provides private data streams which allow such information to be transmitted together with the compressed signal. This has the disadvantage that additional transmission bandwidth is needed for transmitting information about the commands.
2. Description of the Related Art
It has been known to provide the user terminal with means for overlaying on-screen display information over the image received from the image source device. This encompasses for example showing a bar representing the selected volume, showing menus of possible channel selections in which the actually selected item is highlighted, or showing images from the selected channel as a small xe2x80x9cpicture-in-picturexe2x80x9d inside a currently displayed image.
Another application of interactive on-screen display is in games in which a user command dependent foreground item, such as an image of a puppet controlled by the user is super-imposed on a user-independent background such as a landscape through which the puppet is moved under control of the user commands.
It is an object of the invention to provide for a method and device which makes it possible to show information interactively on a user terminal of an image display system, without requiring additional bandwidth for transmitting feedback information.
The invention provides for a method of operating an interactive image display system, wherein the image display system comprises an image source device and a user terminal connected to each other via a link, the user terminal having a user control input, the method including the steps of
receiving an input image signal representing an input image,
receiving a user command from the user control input,
generating a further image signal, dependent on the user commands the further image signal representing an image part,
forming a compressed image signal representing an output image corresponding to the input image with the image part superimposed thereon in the image source device,
transmitting the compressed image signal from the image source device to the user terminal via the link, for decompression of the compressed image signal and subsequent display of the output image, both at the user terminal. Thus interactive information is superimposed on the image at the side of the image source device and transmitted to the user terminal as one image, comprising both the original input image and the overlayed parts. Hence, on the whole no more bandwidth is required than for transmitting the input image. The method is particularly useful for image source devices that are located remotely from the user terminal, and that serve a plurality of user terminals with input image signals from a common source such as a magnetic disk, and that individualize the compressed images transmitted to individual user terminals by overlaying user terminal specific image parts.
In an embodiment of the method according to the invention in said receiving step, the input image signal is received in compressed form and further comprising the step of converting the input image signal into an at most partially decompressed input image signal. The forming step includes forming the compressed image signal by combining the at most partially decompressed input image signal and further image signal, whereby a required amount of computation for forming the compressed image signal from the input image signal is less than a further amount of computation that would be required for forming the compressed image signal from an uncompressed representation of the output image.
An additional advantage is that there are no jitter problems in positioning the image parts relative to the input image, such as occur in systems where the image parts are added to an input signal that is received as an analogue signal containing sync information.
It is to be noted that the operation of superimposing image parts on a compressed image without fully decompressing it is known per se, from a publication by B. C. Smith and L. A. Rowe, titled xe2x80x9cAlgorithms for Manipulating Compressed Imagesxe2x80x9d in IEEE Computer Graphics and Applications (September 1993) pages 34-42, which is included by reference.
In a further embodiment of the method according to the invention the compressed image signal formed in said forming step contains a collection of compressed output blocks, each compressed output block representing a respective subimage of the output image, the collection including first compressed output blocks and at least one second compressed output block. The first compressed output blocks are copied identically from corresponding compressed input blocks from the at most partially decompressed input image signal, the at least one second compressed output block being formed in dependence on the further image signal. Thus the work required for forming the compressed image is limited because only the affected blocks need to be decoded and encoded in anyway at all; the other blocks remain unaffected. Preferably, the blocks are encoded using a Digital Cosine Transform (DCT) as in the MPEG and JPEG standards.
In an embodiment of the method according to the invention the image part is a graphical symbol, with a shape selected according to the user commands.
In a further embodiment of the invention said generating step includes selecting the compressed representation of the graphical symbol from among a set of compressed representations of alternative graphical symbols. Thus most of the computational work for forming the compressed image can be done prior to receiving the user commands. Reaction to the user commands can therefore be faster.
In a further embodiment of the invention the method further includes setting a selectable parameter pertaining to the input image, such as selection rum a number of alternative images, alternative language channels, brightness values, contrast values or sound volume values in dependence of the user command.
In an embodiment of the invention the generating step includes receiving a source image signal, the further image signal being generated by downsizing the further image. This is particularly useful for generating a picture in picture (PIP) display, showing one video channel as a relatively small window inside another video signal. The computational effort required for generating the downsized image in compressed form is less than required for compressing a full image.
The invention also provides for an image source device, having
an image input for receiving an input image signal representing an input image,
a command input for receiving user commands,
generating means for generating a further image signal, dependent on the user command
superimposing means for forming a compressed image signal representing the image with the image part superimposed thereon,
an external output, and
transmitting means, for transmitting the compressed image signal via the external output. This device implements the method.
The image source device can be used remotely, but also locally, being for example a videorecorder or a tuner that is connected only to one user terminal, for example a television monitor. In a system with more than one source device (video recorder, tuner etcetera), on screen display from each of the source devices can be transmitted to the user terminal (the monitor), without requiring extra bandwidth, and without decompression means capable of decompressing full images real time in all image source devices (tuner, video recorder etcetera). Only one such means is needed, in the user terminal (the monitor).